Tuned radio frequency circuits



' D. R. LOVEJOY TUNED RADIO FREQUENCY CIRCUITS Filed June 4, 1925 MIN] AAA

GRID 0/115 WITNESS INVENTOR k QM MAW 5 tably oscillate and atente Nov. a9, 1927.

I I M 1,650,898 STATES PATENT oFFI'CE 'UIIMMITT BOSS LOVEJDY, OF NEW,YORK. Y., ASSIGNOB T LOVEJOY DEVELOPIIIENT CORPORATIQN, OF NEW *YORK, N.

Y., A CORPORATION OF NEW YORK.

TUNED namo ranounnov cr'ncurrs.

Application filed June 4, 1925. Serial No. 34,890.

This invention relates to means for pre'-;

venting undesired regeneration in radio apparatus using triode tube amplification, and particularly to receiving sets having one or more stages of tuned radio frequency-amplification. The object of this invention is to provide means for convenient and exact con trol of the regeneration which usually occurs when such amplification is attempted.

It is well known that radio frequency amplification stages tend to oscillate owing to the various energy feed-backs which are inevitably present. The total value of these energy feed backs varies in each individual. set even in sets which seem tobe identical and even in the same set with different tubes.

These constants may be termed the feed back characteristic of the set. The factors which are involved in this feed back characteristic are the amplifying power of the tubes used, temperature of the filament,\ potential of the grid, potential of the plate and the capacitative and magnetic coupling between the input and output circuits of one or more tubes. In this connection it is convenient to use the expression base line voltage to indicate that fixed voltage about whose value the alternating complement varies. It is also well known that excessive regeneration may be prevented by giving the grids of the radio frequency tubes a positive bias with respect to the negative end of the tube filaments. The amount of such positive bias required is not constant but in a given set varies with 3% the frequency which is being amplified, a greater positive bias being required as the frequency increases. A well known means of impressing the proper grid potential consists in the use of a potentiometer, connected W across the A-battery,

of which is connected the filament end of the grid circuit, the position of the contact being waried empirically and manually so asv to give the correct grid bias when the circuits are tuned to a given frequency. This, of course, necessitates anadditional adjustment in the process of tuning and since it is practically impossible to predetermine the setting ofthe potentiometer, the set will ineviis not great enou gh or else'the set will be sluggish, insensitive, inefiicient and tune broa ly if the positive bias is too great.

. In general the setting of a potentiometer E5 as used in'the above way, bears no relation trol, as by mechanical interconnection,

to the moving contact squeal if the positive bias.

ception and to the annoyance of all radio neighbors.

The necessity of varying the grid bias for differentfrequencies naturally suggests the desirability of conjoint movement of the potentiometer arm with the tuning control. But due to the varying feed-back characteristic of each individual set, and indeed with difi'erentctubes-of the same type, the problem is more complex, I have found, however, that the individual feed back characteristic referred to can be compensated for by giving to the potentiometer a voltage gradlent corresponding to the feed back characteristic ofthe individual set and tubes at a constant filament temperature, and once this gradient has been adjusted and fixed for that particular set the satisfactory con'oint operation of the potentiometer arm an the tuning clpnt en becomes possible and when so connected maximum efliciency of the receiving set without the possibility of squealing will be as- I sured.

The feed back characteristic varies as above stated with the filament temperature." Thus, the filament current would disturb the established coordination above described. Therefore, in receiving sets in wh ch provision is made for varying the filament current, it is necessary to rovide additional means to compensate for the changes in the feed back characteristic caused by changes in the filament temperature. a

My invention comprises means for electrically coordinating the potentiolneter graclient to the its settings correspond wlth the settings of the tuning means, to'the end that when the two controls are connected and operated by a single knob, the position of the potentiometer contact will be just right to give the pro er grid bias for the fr uency determine by that setting of t e tuning means, thus making it posslble for a nun control to give the roper grid blas for all frequencies, hence nsuring the quiet, eficient operation of the set. The means emloyed comprise first-a grid. potentiometer m combination with means for producmg or providing a desired potential gradient therein. This special potentiometer preferably has its contact arm mounted on the shaft of the tuning control. The potential gradient is so adjusted that the potential at the point of contact is correct at that particular setting of the tuning control. Thus, as the circuit is tuned, thegrid bias varies automaticall becoming more positive with respect to t e negative terminal of the tube filament as the frequency increases, and becoming less positive as the frequency decreases. In sets in which the filament temperatures may be varied, additional means are provided for compensating for the changes in the feed back characteristic due to changes in filament temperature.

The invention will be more 'easily understood by a description illustrated bythe accompanying drawings, in whi ch- Figure 1 is a diagrammatic view of a radio circuit.

Figure 2 is a similar view showing additional features, and

Figure 3 illustrates graphically the relation of grid bias characteristics to condenser settings and filament voltages.

In the drawings, C, D and E represent the usual tuning condensers of a 3-tube set having two stages of tuned radio frequency amplification. T is the moving contact of a potentiometer F G, said moving contact bemg suitably mounted upon the shaft of one of the tuning condensers, as by the extended shaft S of the condenser C. When this condenser is turned in the process of tuning, the-contact T moves simultaneously therewith and changes, its point of contact upon potentiometer F G.

The grid circuits of C and D are connected to the contact T, soethat if the potential gradient in the potentiometer is just right, the contact T Wlll rest upon a point whose potential is of the correct value for the feed back characteristic at that particular setting of the condenser.

In order to adjust the end values of the I potential gradient in F G, I prefer to connect it through adjustable contacts J K to a second potentiometer H I which in turn has impressed upon it the potential of the A.- battery.

By suitably adjusting the contacts J and K, anypotential within the limits of the A- batterg may be impressed upon F G, the two ends t ereof being se aratel adjustable. If the resistance of F G is uni ormly distributed, the potential gradient therein will alsb be umform, i. e. a straight line characteristic. But the changes of rid bias r uire l to prevent oscillations at ifierent con enser settings are not necessarily pro ortional to the said condenser settings. Therefore, in order that the potential gradient in F G may be properly related to the condenser settings,

it is necessary in some cases that the resistance of F G be not uniformly distributed but pro erly graduated. This may be accomplis ed by making the resistance unit F G with a non-uniform winding, but I prefer to use av uniform winding and to vary the potential gradient therein by shunting portions of said uniform winding by additional parallel resistances L, M and N of proper values to vary the potential gradient in a way to conform to the grid bias required in a given case. If desired, the resistance F G may be. left uniformly distributed and the condenser settings made to conform to a straight-line relationship by suitably shaping the lates thereof and the same end attained. s the amount of grid bias required to prevent oscillations is not the same at different filament temperatures, less being required as the temperature increases, as shown in Figure 3, it is desirable to provide means for compensation for this variation. In the articular case shown in Figure 3, which is t e result of experimental test of a certain definite arrangement of circuits, the grid bias characteristics are nearly straight-lines between condenser settings 10 to and have the same shape for different filament voltages, being practically parallel and separated in this particular case by a practically fixed difference of .15 volt for .5 volt-change in filament potential.

In an arrangement of circuits such as the above, it is clear that a given change in the filament voltage requires a quite definite chan e in the grid bias to conform to the feed ack characteristic and that the amount of this change within limits is nearly independent of the condenser setting. In other 1 having a movable contact W which is pref- 1 erably mechanically connected, as shown by the dotted line S to the contact V of the filament rheostat Q R so as to be operated synchronously therewith. As the contact V moves to the left to increase the filament 1 current, the contact W also moves to the left and so includes in the grid return circuit less positive bias from the potentiometer X Y, and therefore the grid has characteristic as shown in Fi ward without changing its shape.

With the potential adient in X Y properly adjusted with re erence to the effect of motion of contact V on the filament voltage and to the corresponding position of com 1 re 3 is moved down- 1 tact W, it'will be readily seen'that the required fixed difl'erence in the grid bias characteristics is easily obtained and such difference is independent of the change produced by the motion produced by contact T of potentiometer F G.

I claim 1. In a'tuned radio frequency amplifying circuit using an audion and atuning ele' ment, means for automatically varying at different frequencies the value of the baseline voltage of the grid in a direction and to an extent suitable to offset the change of the remaining factors of the feed back characteristic occurring at such change of frequencies, for the purpose of maintaining maximum efficiency throughout the frequency range, said means comprising a potentiometer having its movable contact connected electrically to the grid and mechanically interconnected with the tuning element and a second potentiometer for adjusting the potential gradient of the first mentioned potentiometer connected across a source of potential which in turn is connected to the filament.

2. In a tuned radio frequency amplifying circuit using an audion, a tuning element and a filament rheostat, means for automatically varying at difierent frequencies its movablecontact connected electricallyto the grid and mechanically interconnected with the tuning element and a second potentiometer for adjusting the potential gradient of the first mentioned potentiometer, connected across a source of potential which in turn is connected to the filament, and a separately energized potentiometer actuated by the filament rheostat and connected with the first mentioned potentiometer, for varying the grid potential determined by the setting of said first mentioned potentiometer, so as to add to or subtract from the potential obtained therefrom an amount equal to the change in the grid potential required by the change in filament current, for the purpose of maintaining maximum efiiciency throughout the frequency range.

DIMMITT ROSS LOVEJOY. 

